fix(fortress): Fixed ConcurrentModificationException when spawning castle ambassadors

src/main/java/com/l2jserver/gameserver/instancemanager/FortManager.java

@@ -18,23 +18,22 @@
  */
 package com.l2jserver.gameserver.instancemanager;
 
-import java.util.ArrayList;
-import java.util.List;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
 import com.l2jserver.commons.database.ConnectionFactory;
 import com.l2jserver.gameserver.InstanceListManager;
 import com.l2jserver.gameserver.model.L2Clan;
 import com.l2jserver.gameserver.model.L2Object;
 import com.l2jserver.gameserver.model.entity.Fort;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 public final class FortManager implements InstanceListManager {
 	
 	private static final Logger LOG = LoggerFactory.getLogger(FortManager.class);
 	
-	private final List<Fort> _forts = new ArrayList<>();
+	private final List<Fort> _forts = new CopyOnWriteArrayList<>();
 	
 	public int findNearestFortIndex(L2Object obj) {
 		return findNearestFortIndex(obj, Long.MAX_VALUE);